Farm tractors typically operate in a two wheel or a four wheel mode of operation. When pulling heavy agricultural equipment through a field, it is often beneficial to utilize a four wheel mode of operation. In other circumstances, such as travel over highways and the like, it is beneficial to condition the tractor in a two wheel mode of operation. Agricultural tractors which automatically disengage the four wheel drive when the tractor ground speed exceeds nine miles per hour are known in the art. On the other hand, four wheel assisted braking for relatively high speed (greater than nine miles per hour) tractors is required in Europe. European standards further require that the tractor default into a four wheel drive condition.
From published German patent DE No. 2,946,477, it is known to engage a front wheel drive clutch on a tractor to transmit power to a front axle when the service brakes for the rear axle are applied. As will be appreciated, however, engagement of the front wheel drive clutch in response to brake application can cause some tractor handling problems when the brakes are applied at high speeds. Known tractor control systems which, in response to tractor braking, lock the clutch assembly to transfer full power to the front wheels as the tractor operates at relatively high speeds creates a condition analogous to locking up the brakes on an automobile. The handling problems inherent with such situations are well known.
Application of the front wheel drive clutch is further complicated in tractors wherein it is often advantageous to use only one of the left or right brakes to assist in steering the tractor through a headland turnaround. With the front wheel drive clutch engaged during a brake-assisted steering operation, more braking effort is required, and front tire pushing, skidding, and wear is likewise increased.
Thus, there is a need and a desire for a drive system which improves braking performance of the tractor especially in cases where the tractor is operated in a two wheel mode of operation and at relatively high speeds (in excess of nine miles per hour).